With snorkeling, you use a snorkel, which is a curved tube that goes from the mouth to above the water so you can breathe. You hold your breath and you can dive deeper under water. With scuba diving, you have oxygen tanks on your back and you can spend the entire time under water.

To add to Mojave's answer, "SCUBA" is an acronym for "Self-Contained Underwater Breathing Apparatus." Self-contained means it does not need an outside source of air. A snorkel has no air supply and draws air from the surface. And a snorkel does not truly let you breathe underwater, or at least not under more than a few inches of water.